This NIH R01 proposal is an integral part of a career commitment by the applicant to elucidate fundamental molecular mechanisms of brain development, by characterizing DNA-binding DNA-binding transcription factors that serve as developmental switching molecules in the nervous system. Understanding these basic principles will in turn advance our understanding of many neurological and behavioral illnesses which have a genetic or developmental component. We have been engaged in studies of Brn-3.0, a member of the POU-domain class of transcription factors. Brn-3.0 is expressed too late in development to act as a regional patterning signal along the neural axis. Instead, its expression pattern indicates a role in the terminal differentiation of specific neurons in the CNS and peripheral sensory system. The distinct anatomical boundaries of Brn-3.0 expression, and the response of Brn-3.0 to patterning signals from the notochord, suggest that it is a downstream target of certain neuroepithelial patterning genes. Brn-3.0 expression is also maintained throughout development, in spite of the fact that some Brn-3.0 neurons mitigate long distances from their neuroepithelia of origin to their final nuclear locations. This "locked in" pattern of expressions suggests that Brn-3.0 may positively regulate its own enhancer domains once expression has been initiated by patterning factors. W have shown by targeted expression of beta-galactosidase in transgenic mice that the regulatory information which determines the specific pattern for Brn-3.0 expression lies within 6.0kb of its transcriptional start site. This proposal will address the regulation of Brn-3.0 expression through four Specific Aims: 1) Further define the functional domains of the Brn-3.0 promoter by targeting of beta- galactosidase expression in transgenic mice. 2) Examine the expression of Brn-3.0 and the activity of the Brn-3.0 promoter in mice which lack functional Pax-3 and Pax-6, two patterning genes which are candidate regulators of Brn-3.0 expression. 3) Assess regulation of the Brn-3.0 promoter by specific homeotic regulatory genes and by Brn-3.0 itself in cell transfection studies. 4) Determine the extent of Brn-3.0 autoregulation genetically, by creating hybrid mice which expression beta-gal under control of Brn- 3.0 regulatory sequences, but are null for expression of Brn-3.0 itself, derived from existing mouse mutants.